Chapter 14: The Lymphatic System and Immunity – Study Notes

Chapter 14: The Lymphatic System and Immunity

Introduction to the Lymphatic System

The lymphatic system is a network of cells, tissues, and organs that work together to defend the body against pathogens—disease-causing organisms. It plays a crucial role in maintaining fluid balance, absorbing dietary fats, and providing immune responses.

• Pathogen: Any organism that causes disease (e.g., bacteria, viruses, fungi, parasites).

• Lymphocytes: The primary cells of the lymphatic system, responsible for immune responses.

Immunity

Immunity is the body's ability to resist infection and disease. It is divided into two main types:

• Innate (Nonspecific) Immunity: Provides general defense against pathogens through physical and chemical barriers (e.g., skin, mucous membranes, phagocytes).

• Adaptive (Specific) Immunity: Involves lymphocytes that respond to specific pathogens. This is also called the immune response and includes T cells and B cells.

• Cell-mediated immunity: T cells attack pathogens inside living cells.

• Antibody-mediated immunity: B cells produce antibodies that target pathogens in body fluids.

Components of the Lymphatic System

The lymphatic system consists of four main components:

• Lymphatic vessels (lymphatics): Transport lymph from peripheral tissues to veins.

• Lymph fluid: Fluid similar to plasma, found within lymphatic vessels.

• Lymphocytes: Specialized white blood cells (T cells, B cells, NK cells).

• Lymphoid tissues and organs: Includes lymph nodes, thymus, and spleen.

Functions of the Lymphatic System

• Production, maintenance, and distribution of lymphocytes to fight threats through physical and chemical actions.

• Returns excess fluid and solutes from peripheral tissues to the bloodstream, maintaining fluid balance.

• Distributes hormones, nutrients, and waste products into general circulation.

Lymphatic Ducts

• Thoracic duct: Drains lymph from the lower body and left side of the upper body into the left subclavian vein.

• Right lymphatic duct: Drains lymph from the upper right side of the body into the right subclavian vein.

Lymphocytes and Their Types

• T Cells: Regulate and carry out attacks on foreign bodies.

  • Cytotoxic T cells (CD8): Directly attack and destroy foreign cells.

  • Helper T cells (CD4): Stimulate activities of T and B cells.

  • Suppressor T cells: Inhibit activities of T and B cells to prevent overreaction.

  • Regulatory T cells: Include both helper and suppressor functions.

• B Cells: Make up 10–15% of lymphocytes. Differentiate into plasma cells that secrete antibodies (immunoglobulins) for antibody-mediated immunity.

• Natural Killer (NK) Cells: Make up 5–10% of lymphocytes. Provide rapid responses to virus-infected cells and tumor formation.

Lymphoid Organs and Tissues

• Thymus: Secretes hormones called thymosins, which stimulate cell division and maturation of T cells.

• Lymph nodes and spleen: Filter lymph and blood, trapping pathogens and facilitating immune responses.

Phagocytes

Phagocytes are cells that engulf and digest cellular debris and pathogens.

• Microphages: Include neutrophils and eosinophils. These cells leave the bloodstream and enter infected tissues to phagocytize pathogens.

• Macrophages: Larger phagocytes that reside in tissues and are less mobile. Known as "big eaters."

Cytokines and Interferons

• Cytokines: Chemical messengers released by tissue cells to coordinate local immune activities.

• Interferons: A type of cytokine released by activated lymphocytes, macrophages, and infected tissue cells. They help inhibit viral replication.

Inflammation

Inflammation is a localized tissue response to injury, characterized by swelling, redness, heat, and pain. It is a protective response that helps eliminate the initial cause of cell injury.

• Caused by the release of histamines and heparin from damaged tissues.

• Tissue destruction: Known as necrosis.

• Pus: Accumulation of dead and dying cells at the injury site.

• Abscess: Pus enclosed in a tissue space.

Fever

• Fever: An elevation of body temperature above the normal range, often in response to infection.

• Pyrogens: Proteins that reset the temperature center in the hypothalamus, causing body temperature to rise.

Immunodeficiency Diseases

Immunodeficiency diseases occur when the immune system fails to develop normally or is blocked.

• HIV (Human Immunodeficiency Virus): Leads to a decrease in helper T cells.

• AIDS (Acquired Immunodeficiency Syndrome): Develops when there are not enough helper T cells to activate cytotoxic cells, resulting in severe immune suppression.

Allergies (Hypersensitivity Reactions)

Allergies are inappropriate or excessive immune responses to antigens.

Type	Description
Immediate hypersensitivity	Rapid allergic reaction (e.g., anaphylaxis)
Cytotoxic reactions	Antibody-mediated destruction of cells (e.g., blood transfusion reactions)
Immune complex disorders	Formation of antigen-antibody complexes; slow phagocyte activity
Delayed hypersensitivity	Inflammatory response occurring two to three days after exposure (e.g., contact dermatitis)

Summary Table: Major Cells of the Immune System

Cell Type	Main Function
T Cells	Cell-mediated immunity; attack infected or abnormal cells
B Cells	Produce antibodies for antibody-mediated immunity
NK Cells	Destroy virus-infected and cancerous cells
Phagocytes (Neutrophils, Macrophages)	Engulf and digest pathogens and debris

Key Terms and Definitions

• Antigen: Any substance that can trigger an immune response.

• Antibody (Immunoglobulin): Protein produced by B cells that binds to specific antigens.

• Cytokine: Chemical messenger that regulates immune responses.

• Necrosis: Death of tissue.

• Pus: Accumulation of dead cells and fluid at an infection site.

Example: When a person is infected with the influenza virus, the innate immune system responds first with barriers and phagocytes. If the virus enters cells, cytotoxic T cells target and destroy infected cells, while B cells produce antibodies to neutralize the virus in body fluids.

Additional info: The lymphatic system also plays a role in absorbing fats from the digestive tract via specialized lymphatic vessels called lacteals.